Heat Activated Transfers are used for a variety of different applications, including providing ornamental indicia on garments, providing lettering on sports jerseys, and providing identification on other types of cloth and materials. Although formed from a variety of different materials, they are generally laminates with a lower adhesive layer and an upper pigmented layer which provides the indicia. There are other substitute products and methods such as screen printing wherein a pigmented, polymeric material is printed directly onto a substrate and cured.
One very successful heat-activated transfer laminate is disclosed in Mahn U.S. Pat. No. 4,269,885. This product is a laminate with a lower polyester adhesive layer and an upper pigmented polyurethane or polyether layer. One of the problems with laminates such as this is that over time, and during application in particular, they tend to curl. The curling makes it more difficult to apply these uniformly.
The invention disclosed in the Mahn '885 patent overcame this problem by forming a laminate where the upper and lower lamina had a compatible or similar coefficient of thermal expansion. Thus, upon heating, both layers reacted in the same manner which reduced or prevented curl. In order to achieve this, the upper and lower layers must be carefully matched.
However, it is frequently desirable to modify the characteristics of either the polyester or the polyurethane layer. For example, if one were to apply a letter on a letter one slightly smaller than the other with contrasting colors, one would need to adhere the polyester layer of the upper letter to the polyurethane layer of the lower letter (which is adhered to the cloth substrate). The product disclosed in the Mahn '885 patent does function in this manner, but has some problems. The upper layer generally will include an external lubricant which, over time, will bloom and interfere with the adhesion between the upper letter and the lower letter. Further, functionally the product tends to be very soft. If the lower letter is very dark, it tends to bleed through the upper letter, presenting an unpleasant appearance.
The bleedthrough could be solved by simply changing the durometer of the pigmented layer. But when this is done, it is extremely difficult to match the adhesive layer to the pigmented layer to prevent curling. Likewise, the external lubricant can be removed. But again, this change makes it extremely difficult to match the adhesive layer to the pigmented layer, again resulting in curling.